1. Field of the Invention
The invention relates to a coupling structure for coupling a shaft to a universal joint yoke, a coupling method for coupling a shaft to a universal joint yoke, and an intermediate shaft.
2. Description of the Related Art
In an automobile, generally, a steering shaft coupled to a steering wheel that serves as a steering member and, for example, a pinion shaft that serves as a steering gear shaft are coupled to each other via an intermediate shaft. As such an intermediate shaft, a telescopic shaft is usually used. The telescopic shaft is formed by coupling an inner shaft and a tubular outer shaft to each other such that the inner shaft and the outer shaft are slidable relative to each other in the axial direction. An end portion of each of the inner shaft and the outer shaft of the intermediate shaft is coupled to a corresponding one of the steering shaft and the pinion shaft via a universal joint.
Conventionally, a yoke of each universal joint is integrally fixed to a corresponding one of shafts of an intermediate shaft by welding. According to Japanese Patent Application Publication No. 2012-112509 (JP 2012-112509 A), a yoke and a corresponding one of shafts of an intermediate shaft are fitted to each other by serration-fitting so that steering can be performed even if a welded potion is broken. Further, in order to prevent the shaft from coming off the yoke, a riveted portion, which is formed by expanding an insertion pipe-shaped distal end portion of the shaft outward in the radial direction, is provided so as not to be in contact with the yoke .
When the yoke and the shaft are fitted to each other by meshing their serration teeth with each other, it is possible to make only stepwise rough adjustment at every pitch of teeth, for example, at intervals of 10 degrees. For example, when an angular position at which one of the universal joint yokes is fitted to one end of the shaft is changed to adjust the phase between the one of the universal joint yokes and the other universal joint yoke fixed to the other end of the shaft, it is possible to make only stepwise rough adjustment. Therefore, the number of design variations of the intermediate shaft, which can be realized with the use of common parts, is limited. Therefore, it is not possible to obtain a mass production effect. As a result, the overall production cost increases.
When a failure, for example, a breakage of the welded portion occurs, it is still possible to transmit torque through the intermediate shaft because the serration teeth of the intermediate shaft and the serration teeth of the yoke mesh with each other. Thus, a fail-safe is achieved. However, a driver does not notice occurrence of the failure immediately after the failure occurs, and recognizes occurrence of the failure only after a slip of the serration teeth or the like occurs.